The present invention relates to a novel process design for removing acid gases from natural gas and other gas streams. In particular, it relates to a process for increasing the selectivity and capacity for hydrogen sulfide removal from a natural gas stream using amine absorbents.
A number of different technologies are available for removing acid gases such as carbon dioxide, hydrogen sulfide, carbonyl sulfide. These processes include, for example, chemical absorption (amines, including alkanolamines), physical absorption (solubility, e.g., organic solvent, ionic liquid), cryogenic distillation (Ryan Holmes process), and membrane system separation. Of these, amine separation is a highly developed technology with a number of competing processes in hand using various amine sorbents such as monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), N-methyldiethanolamine (MDEA), diisopropylamine (DIPA), diglycolamine (DGA), 2-amino-2-methyl-1-propanol (AMP) and piperazine (PZ). Of these, MEA, DEA, and MDEA are the ones most commonly used.
It is often necessary or desirable to treat acid gas mixtures containing both CO2 and H2S so as to remove the H2S selectively from the mixture while minimizing removal of the CO2. While removal of CO2 may be necessary to avoid corrosion problems and provide the required heating value to the consumer, selective H2S removal may be necessary or desirable. Natural gas pipeline specifications, for example, set more stringent limits on the H2S level than on the CO2 since the H2S is more toxic and corrosive than CO2: common carrier natural gas pipeline specifications typically limit the H2S content to 4 ppmv with a more lenient limitation on the CO2 at 2 vol %. Selective removal of the H2S may enable a more economical treatment plant to be used and selective H2S removal is often desirable to enrich the H2S level in the feed to a sulfur recovery unit.
FIG. 1 shows a schematic representation of a typical absorption-regeneration unit used for the removal of H2S and CO2. The acid gas scrubbing process using an amine in the purification process usually involves contacting the gas mixture countercurrently with an aqueous solution of the amine in an absorber tower. The liquid amine stream is then regenerated by desorption of the absorbed gases in a separate tower with the regenerated amine and the desorbed gases leaving the tower as separate streams. The various gas purification processes which are available are described, for example, in Gas Purification, Fifth Ed., Kohl and Neilsen, Gulf Publishing Company, 1997, ISBN-13: 978-0-88415-220-0.
U.S. application Ser. No. 14/980,634, which is incorporated by reference in its entirety herein, describes an absorbent system that can selectively absorb H2S from gas mixtures that also contain CO2 by controlling the pH of the absorbent system. The pH of the amine/alkanolamine absorbent system is reduced/controlled in one aspect by diluting the concentration of the amine/alkanolamine absorbent. The lower pH favors bicarbonate formation, increasing acid gas (H2S and CO2) loading, and increasing the selectivity of H2S over CO2 over a broad loading range. Particular amines that are found useful in the present invention are amines and alkanolamines, preferably sterically hindered amines and alkanolamines, and most preferably capped, sterically hindered amines such as methoxyethoxyethoxyethanol-t-butylamine (M3ETB).
FIG. 1 shows a schematic representation of a typical absorption-regeneration unit used for the selective removal of H2S and CO2. The acid gas scrubbing process using an amine in the purification process usually involves contacting the sour gas mixture 10 countercurrently with an aqueous solution of the lean amine 12 in absorber tower 100, producing sweet gas mixture 11. The rich amine stream 14 is then regenerated by desorption of the absorbed gases in regenerator tower 110 with the regenerated lean amine 12 and the desorbed gases 16 leaving regenerator tower 110 as separate streams. This and other gas purification processes that are available are described, for example, in Gas Purification, Fifth Ed., Kohl and Neilsen, Gulf Publishing Company, 1997, ISBN-13: 978-0-88415-220-0.
What is still needed is a process design for efficiently and effectively taking advantage of the teachings of U.S. application Ser. No. 14/980,634, specifically the improved selectivity of H2S over CO2 by reducing/controlling the pH of the absorbent system.